1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method for polishing a substrate such as a semiconductor wafer, and more particularly to a polishing apparatus suitable for use as a bevel polishing apparatus for polishing a bevel portion of a substrate and as a notch polishing apparatus for polishing a notch portion of a substrate.
2. Description of the Related Art
From a viewpoint of improving a yield in semiconductor fabrications, management of a surface condition in a periphery of a semiconductor wafer has recently been drawing attention. In semiconductor fabrication processes, a number of materials are deposited on a wafer repeatedly to form multilayer structures. As a result, unwanted films and a roughened surface are formed on a periphery of the wafer which is not used for products. In recent years, it has become more common to transfer the wafer by holding only the periphery of the wafer with arms. Under such circumstances, the unwanted films could come off the periphery onto devices formed on the wafer during several processes, resulting in a lowered yield. Thus, it is conventional to polish the periphery of the wafer using a polishing apparatus so as to remove the unwanted films and the roughened surface.
A polishing apparatus using a polishing tape for polishing a periphery of a substrate has been known as such a type of polishing apparatus. This type of polishing apparatus polishes the periphery of the substrate by bringing a polishing surface of the polishing tape into sliding contact with the periphery of the substrate. Since a type and a thickness of an unwanted film to be removed vary from substrate to substrate, multiple polishing tapes with different roughness are generally used. Typically, rough polishing is performed so as to remove the unwanted film and form a shape of the periphery, and then finish polishing is performed so as to form a smooth surface.
A bevel portion and a notch portion are generally formed in the periphery of the substrate. The bevel portion is a part of the periphery where angular edges have been removed. This bevel portion is formed for the purpose of preventing the substrate from being cracked and preventing production of particles. On the other hand, the notch portion is a cutout portion formed in the periphery of the substrate for the purpose of specifying a crystal orientation. The above-described polishing apparatus for polishing the periphery of the substrate can be classified roughly into a bevel polishing apparatus for polishing the bevel portion and a notch polishing apparatus for polishing the notch portion.
Examples of the conventional bevel polishing apparatus include a polishing apparatus having a single polishing head and a polishing apparatus having multiple polishing heads. In the polishing apparatus having a single polishing head, multistage polishing is performed by replacing a polishing tape with another polishing tape having a different roughness after polishing or by transferring the substrate from a rough-polishing section to a finish-polishing section. On the other hand, in the polishing apparatus having multiple polishing heads, rough polishing and finish polishing can be performed successively.
However, in these conventional apparatuses, a long polishing time is required as a whole, because finish polishing is performed after rough polishing. Specifically, the total polishing time is the sum of a rough-polishing time and a finish-polishing time. In addition, the polishing tape needs to be replaced with a new polishing tape periodically, because the polishing tape is a consumable part. Therefore, there is a demand for easy operation for replacing the polishing tape as a consumable part, and there is also a demand for use of as long a polishing tape as possible in view of reducing frequency of the tape-replacement operations.
On the other hand, as disclosed in Japanese laid-open patent publication No. 2005-252288, a polishing apparatus configured to press plural polishing tapes with different roughness against the periphery of the substrate successively is known as a conventional notch polishing apparatus. However, in this conventional apparatus, polishing heads are close to each other and this arrangement makes it difficult to conduct maintenance of the polishing heads. In addition, since reels each containing the polishing tape are adjacent to each other, it is difficult to replace the polishing tape. As a result, a polishing time including the replacement time of the polishing tapes becomes long.